The phenomenon of the River Festival is as old as time, but the origin and deeper significance are generally unknown—as is its inherent danger. 
We are all aware of those strange tides, when a large river of a certain, particular depth and structure, occasions to draw an abnormally large amount of water from the banks of the river, revealing queerly preserved things, many of which are of unidentifiable antiquity. A town built on the banks of the river will have its people awake—for the uncommon tides which both reveal and bring Festivals occur only at daybreak—to find the more ancient parts of their settlement encased, almost mummified, in a layer of black slime, having been hidden just beneath the surface of the river. Why no one has ever had his vessel blocked by these ghosts of prior incarnations of the town, or any swimmer found his legs grazed by the submerged remnants of what once was—no one ever seems to wonder.
All questions, were they to be asked, would be hushed with the arrival of a motley flotilla of boats and barges, from which the most alien music can be heard. Come to set up their Festival on the grounds of the recently revealed Old Town are the clowns and acrobats and soothsayers of the Festival. They are an uncanny folk: Thin and far too tall, even without stilts—sometimes nearly twice the height of a normal man; flexible in ways that seem impossible, their limbs and spines cracking as they bend; and most uncanny is the fact that while they laugh, cry, and make apparently normal human sounds, they never speak. Not a single word escapes those uncanny folk for the entire duration of the Festival.
They set up their tents and tables of wares—the likes of which cannot be found anywhere else in the world; indeed, it is at times unclear what their wares even are—with such a speed and dexterity which indicate how many times they must have done so: Thousands upon thousands of times, presumably passed down from one generation to the next—though it is unclear how old any of the uncanny folk are, so heavily smeared as they are in their comical war paint. One is given the distinct impression, indeed, that these folk have set up their tents and the like in very precise, even rehearsed, positions. That is to say, that they were wont to appear thousands of years ago, when the Old Town, now uncovered by the strange tide, was above water—a time before the town had the name it has, if it even had one.
The Festival lasts only as long as the tide is out, though the low tide is more protracted than any other. The uncanny folk dance, laughing and crying and laughing again, in circles around various booths which are manned, as it were, by shorter, but far more slender, vendors, whose faces are painted darker colors—more blues and purples and not the reds and blues yellows of the dancers. Almost nothing is ever bought or sold, as it is never clear what sort of money the Festival folk use, but occasionally there is a trade. A cow, though sick, will go for a set of what appear to be thirty bejeweled rings. Coarse rye bread, though riddled with ergot, will be taken in exchange for a silver crown, though one too large for anything which could be called a head.
There are always three tents erected. One functions as a sort of “entrance,” though it is not physically connected to the other two. Within it are specialized acrobats. There are some who one might find doing two flips, a front and then a back, before landing on the ground. There are contortionists who go so far as to remove limbs in their excessive twisting, though this must involve some sleight, as the limbs regain function when replaced. There are long-necked men who give dramatic dances upside down, using only their heads as support. There are women in oddly-flowing clothing who jump 30 or 40 feet into the air and seem even to hover. And there are others, of course, for the human form is hugely malleable, and who knows of what the uncanny folk are capable. 
Outside, between the tents, there are odd, black stone steles set up which ostensibly give an explanation of the Festival, but the etchings are worn far beyond readability. If one stops for too long in front of these steles, trying to make sense of the runes and scratches, or simply gawking, a roundishly-dressed clown will come waddling over to guide you along. 
A second tent is where the soothsayers and other mystics and mentalists are set up—necromancers and face readers and stranger clairvoyants which have neither name nor obvious function. Little is known about what goes on in the second tent. Most who enter exit with almost no memory of what they experienced and those who do have memories too strange to be real. There is talk of spaces that are “alive” and others who claim to have spoken to God, but that it was not God who responded; some believe that they were transported to impossibly remote places and times, others planets around other stars—or simply earth, but an earth with neither sun nor moon and only a handful of stars to light the mountains, hundreds of miles high, in all directions. 
The third tent is rumored to be empty. Two of the tallest and most slender of the uncanny folk, dressed in clownish uniforms which shine a dark light, painful to look at directly, guard the entrance. No one is allowed in and no one ever emerges. If there is anyone inside, no one has ever seen him. This has not, of course, stopped speculation over the years. This is particularly popular among children who claim that their small size allows them to sneak into the third tent undetected. It has been told that the third tent contains nothing at all and is a dark, empty space. This would raise the obvious question as to why on earth the third tent should exist. (But really, why should there be a tent which exists, but no one is allowed to enter.) Others tell of a “master” clown, or harlequin, whose name varies from locale to locale—Sannei, Zill, Tsomni, and others—but whose role as “master” is never revealed. 
Perhaps the most widespread of the children’s rumors is that the third tent leads to an entirely other world, the world from which the uncanny folk originally came from and visit from time to time. The claim is that they are not human, which is of course easily believable, and come from a place very much like our world, but one in which all of the “people” exist in one, gargantuan and perpetual festival, the scale of which is almost incomprehensible. The people there are even larger and stranger and, the rumor goes, laugh and laugh, never ceasing in their laughing, so wonderful their great festival is. This is likely the most widespread rumor because there is, indeed, a quiet, distant, sound of many overlapping, laughing voices coming from the tent. Or, so it would seem. The guards, while stationed at the entrance, are so tall and quick and flexible that even at the back of the third tent, should anyone venture too closely, a hissing white face with blue-and-red eyes flanked by a pair of black hands is sure to greet the incautious individual. The hissing is a painfully loud and repulsive sound and the hands, the black hands, are blacker than black. They are not like any black surface which emits a black sort of color. They are a black that absorbs light and color and, indeed, it seems, the very thoughts and gaze of the approacher.
The black hands of these titanic harlequins are the very thing of grandmothers’ tales told to keep children from venturing too far from home, from staying out too late at night. But they are also a warning, to keep away and never enter a River Festival—though, of course, they do. And children do go missing, but the uncanny folk of the River Festival are never blamed. Some other explanation is always found. A rogue wolf. A bear. A runaway. But everyone knows that the children have been taken. To where it is is unknown.
And just as quickly as they arrived, the uncanny folk pack up their tables and wares, the tents and obelisks, and depart. With their going, the tide returns, and yet no one ever wonders how it is that the tide could have been out for days at a time.a